This invention relates generally to the growth of semiconductor materials from which devices are fabricated and, more particularly, to a method of monitoring and controlling the deposition of crystalline films of Group III/V compound semiconductors by molecular beam epitaxy (MBE). Molecular beam epitaxy is a process in which atoms and molecules are evaporated onto crystalline substrates in ultra-high vacuum, i.e., 1.times.10.sup.-9 torr background pressures. Because of the cleanliness of the environment, the resulting crystalline films are near-perfect and have outstanding optical and electrical properties. Appropriate shuttering of the atomic and molecular fluxes, moreover, enable combinations of different alloy compositions and thicknesses to be created and layered, from which optical and electronic devices may eventually be fabricated.
Among the most commonly deposited alloys grown by MBE are various combinations of Group III, e.g., Al, Ga or In, and Group V, e.g., P, As or Sb, elements. The thermodynamics of these alloys requires predominantly a one-to-one ratio between elements of the two groups, e.g., one molecule of Ga will covalently bond with one molecule of As. The surface chemistry of these alloys also stipulates that all incident Group III atoms nearly always "stick" to the growth surface, while incident Group V molecules only "stick" to the growth surface when Group III atoms have already "stuck" to the growth surface. Therefore, it is common practice to bathe the semiconductor substrate surface in a flux rich in Group V molecules because the excess Group V flux, beyond that required to form the stoichiometric (1:1) III-V compound, will simply reflect back from the growth surface. Note that for both Group III and Group V fluxes, it is important to distinguish between the incident flux of atoms or molecules actually incident on the growth surface and the incorporated flux which is that portion of the incident flux that actually "sticks" and incorporates into the growing crystal.
An important class of III/V material deposited by MBE through appropriate choices of incident atomic or molecular fluxes consists of III-III-V alloys in which more than one Group III element is present, e.g., Al.sub.x Ga.sub.1-x As, or III-V-V alloys in which more than one Group V element is present, e.g., InAs.sub.x Sb.sub.1-x. In these cases, by composition, "x", we do not mean the III/V ratio in either a III-III-V or a III-V-V crystal, which is always unity, but rather the III/III ratio in a III-III-V crystal, or the V/V ratio in a III-V-V crystal.
In such ternary alloys, the composition is an important determinant of device performance, and must be controlled accurately. Currently, it is common practice to calibrate the atomic or molecular fluxes before growth, and then to rely on those fluxes to remain stable during growth. Such stability is difficult to maintain especially during complex and extended growth sequences greater than, for example, four hours.
The only existing method for deducing III/III incorporation ratios, i.e., the composition of a III-III-V alloy, or for deducing V/V incorporation ratios, i.e., the composition of a III-V-V alloy, during growth is based on oscillations in reflection high-energy electron diffraction (RHEED) intensities. Such oscillations are known to be periodic with the atomic bilayer by bilayer growth cycle, where each bilayer is a sheet of Group III atoms and a sheet of Group V atoms. The intensity minima in the oscillations occur whenever half an atomic bilayer is completed, and the intensity maxima whenever a full atomic bilayer is completed. Therefore, the RHEED oscillation technique can measure absolute growth rates. Then, from measurements of the rate at which, for example, a III-III-V alloy grows and the rate at which one of the constituent III-V alloys grows (by shuttering off the other Group III source), the III/III incorporation ratio may be determined. The V/V ratio in a III-V-V alloy may be determined in a similar way.
RHEED oscillations, however, are observed only under fairly restrictive conditions. A further disadvantage is that RHEED oscillations can only be initiated on perfectly smooth surfaces, which typically are prepared by long, i.e., greater than five seconds, and undesirable growth interruptions. Monitoring absolute growth rates using RHEED oscillations, moreover, sometimes requires crystal growth at lower-than-optimal temperatures. The RHEED oscillations technique further requires that the substrate be immobilized during growth, which is inconsistent with the preferred and common practice of rotating the substrate during growth to increase lateral uniformity. Finally, measuring RHEED oscillations requires at least and generally much more than a full bilayer of sacrificial growth of one of the pure binary compounds.
Although the RHEED oscillations method is commonly used before growth as a method to calibrate the compositions expected during growth, it remains impractical because of the limitations discussed above and it is not universally useful for real-time determination of III-III-V or III-V-V alloy composition.
Mass spectrometric measurements of atomic or molecular fluxes "reflected" or "desorbed" from the growth surface are sometimes called Reflection Mass Spectrometry (REMS) measurements. REMS measurements usually make use of a cryo-shrouded mass spectrometer that measures mass-analyzed, line-of-sight chemical fluxes from a growing wafer. Previous work of Foxon and Joyce; Arthur; SpringThorpe and Mandeville; Evans, Stutz, Lorance and Jones have shown that REMS measurements provide much information about the growth surface itself. To put these previous studies of REMS measurements in context, they may be classified into one of two categories.
The first category includes the work of Brennan, Tsao, Klem, Hammons and Jones (1989); SpringThorpe and Mandeville; Evans, Stutz, Lorance and Jones and comprises measurements made of Group III fluxes reflected or desorbed from the growth surface. Although these measurements are interesting in their own right, the Group III fluxes leaving the surface are normally quite small. Moreover, the reflected flux of Group III is complexly dependent upon the incident and incorporation fluxes of all other species as well as on temperature. A simple measurement of Group III reflected fluxes, therefore, has not led nor is likely to lead to a technique for deducing the incorporation rate of Group III or Group V species during growth.
The second category of REMS measurements' studies includes the research of Tsao, Brennan and Hammons (1988); Tsao, Brennan, Klem and Hammons (1989); Foxon and Joyce; and Arthur and measure Group V fluxes reflected or desorbed from the growth surface. These reflected Group V fluxes are also an indirect measure of the incident Group III fluxes for the following reason. Under ordinary growth conditions for which the incident Group V flux is greater than the incident Group III flux, every incident Group III atom consumes exactly one Group V atom and decreases the reflected Group V flux by one atom. Therefore, the lower the measured reflected Group V flux, the higher must be the incident Group III flux. Likewise, the higher the incident Group III flux, the lower the measured reflected Group V flux. In principle, from this "inverse" one-to-one relation between the incident Group III flux and the reflected Group V flux it should be possible to absolutely determine the incident Group III flux. But an absolute measurement of the incident Group III flux requires that the absolute incident Group V flux be already known from a previous measurement. In practice, Group V fluxes not only are difficult to calibrate exactly, but the flux actually drifts during long growth runs. Moreover, Group V species are not pumped effectively by the liquid-nitrogen-cooled cryoshrouds in the chamber. Thus as the crystal is grown, the Group V species create a "background" pressure. This background pressure further complicates the inverse one-to-one relation between the incident Group III flux and the reflected Group V flux. Thus, this method of measuring Group V reflected flux has not resulted in a reliable and stable technique for deducing the Group III or Group V fluxes actually incident on the growth surface.